1. FIELD OF THE INVENTION
The present invention relates to noise reducing circuit for video signals for reducing a white noise which might be contained in the video signal.
2. DESCRIPTION OF THE PRIOR ART
The amplitude vs. frequency characteristic of a video signal processing circuit may be emphasized in a high frequency region to make an improvement in the sharpness of a reproduced image. This permits high frequency components of the video signal to be emphasized which are responsible for the contour of reproduced images, thereby improving the sharpness of the reproduced images. The emphasis of the high frequency components, however, causes the simultaneous emphasis of random noise components such as thermal noise contained in the video signal which results in deterioration of the quality of reproduced images. Usually, the emphasis of the high frequency components is limited to such an extent that the random noise components are not remarkable in reproduced images. The reduction of random noise components is, therefore, essential in order to effect a sufficient emphasis of the high frequency components.
Meanwhile, only a very small portion of the entire frame of a reproduced image is occupied by the contour portions of the reproduced image, in which changes of the brightness take place and therefore high frequency components are dominantly involved. Thus, the greater portion of the image is occupied by flat portions, in which no considerable change of the brightness takes place and therefore the low frequency components are dominantly involved. For this reason, an image with the reduced noise can be visually obtained by only reducing the random noise superimposed on the flat portions of the image. Therefore, the greater improvement in the sharpness of the reproduced image can be provided by reducing the random noise superimposed on the flat portion of the image, that is, by reducing the random noise superimposed on that portion of the video signal which doesn't contain high frequency components.
A noise reducing circuit for eliminating random noise superimposed on those portions of the video signal which are responsible for the flat portions of reproduced image is disclosed in Japanese Patent Publication No. 21269/67 published on Oct. 20, 1967. This published application has matured into Japanese Pat. No. 515842 entitled "System for Reducing White Noise in Television Signal or the Like" (Patentee: Japan Broadcasting Corporation). The noise reducing circuit according to this Japanese patent is expensive since it needs a great number of active elements and delay elements.
Since the level of noise contained in the video signal depends upon the intensity of an electric field of an input signal, it is preferable to make the degree of the noise reduction variable so as to lower the level of noise to be eliminated when the video signal has a high signal-to-noise ratio, thereby making smaller the higher frequency components of the video signal which must be eliminated together with the noise.